rainbow loom

Cooper Volkman is 8 years old and full of energy. While his 10-year-old sister sits down easily to focus on projects, Cooper tends to be perpetually in motion. Lately, though, it’s been different. In the evenings, when his mom, Lori, makes dinner at home in the suburbs outside Portland, Ore., Cooper is absorbed by a new hobby: making bracelets and other objects out of tiny, colorful rubber bands. It’s the first craft project that has captured his attention. And his mother says it’s helping him marshal the kind of focus and planning skills previously shown only by his big sister. The current craze for kids—Rainbow Loom bands and their many knockoffs—has been surprising parents and child development experts since it broke out last summer and fall: In a market glutted with c...